This was definatly a plastic cased C/U rather than a type A board, had a C/U type busbar like the one in the picture as opposed to the iso-bar assembly. Just had a look and I haven't got any pictures. It was in a tea room cabin at a warehouse and was I think, possibily the only cabin I've seen that didn't have a 30mA incommer. It defintaly wasn't a problem to fit.
I know the square D boards you mean, interestingly I tested one I took out to see if it was as flamable as the plastic one of another make that you burnt.... it wasn't, went out as soon as heat removed , but then again, it was a bit older too and I think that might have more bearing than the manufactuer (well not age as such, but date of manufacture)
The OP's DB was the same type a large modular/portable building company fitted as standard for many years, the din rail configuration was changed a few years ago where the plastic grid that the MCB's are seated was replaced with a metal DIN rail, the physical size of the DB was unchanged and it didn't affect the fitting of RCBO's.
I've fitted many RCBO's in this type of DB.
This type of RCBO will fit no problems.
https://www.ebay.co.uk/itm/MERLIN-G...560887?hash=item3b2b5c9f77:g:iFAAAOSwGWJcQ7Gg
Links in this post may contain affiliate links for which DIYnot may be compensated.